Bulge

From VVV Survey

Bulge

VVV Survey Area

Most of the stars, gas and dust in the Milky Way are confined to the bulge and plane of the Galaxy. For this reason, extinction and crowding make it difficult to unveil the inner structure of the Milky Way and to study in detail the formation and evolution of this representative galaxy. Traditional distance indicators have been used with varied success in the past. The approach was to concentrate in the clear “windows”, where optical surveys can be carried out (MACHO, OGLE, EROS). With VISTA, it is now possible to map the whole bulge systematically for several epochs in the near-IR. We propose to cover a 300 sq deg area, containing ~5x108 point sources. Our survey will give the most complete catalogue of variable objects in the bulge, with ~106 variables. Chief among them are the RR Lyrae, which are accurate primary distance indicators, being well understood from their chemical, pulsational and evolutionary properties. For the sake of space and coherence we concentrate on the RR Lyrae (goal 1) and the star clusters (goal 2), noting that similar worthy studies can be done for many of the other populations of variable objects (goals 3-10).

Existing, single-epoch near IR surveys (e.g., COBE) have proven that the Galactic Bulge is boxy and contains a bar [1]. Presently, the only model we have for the formation of boxy/barred bulges is via secular evolution of a pre-existing disk. This scenario is believed to be the dominant channel of formation of bulges in late-type spirals (Sbc), whereas early-type spiral bulges (S0/Sa) show structural and kinematic evidence for an early, rapid collapse, which seems to be confirmed by the old age of their stellar population [2].

However, the best studied spiral bulge, in the Milky Way, is precisely the most problematic one to understand in this context. While its surface brightness shows a barred structure, its stellar population is old [3][4] and it has α-element enhancement, characteristic of a rapid formation. Most importantly, the chemical composition of bulge stars is different from that of both thin and thick disk stars [5]. Thus, the formation of the Milky Way bulge via secular evolution of the disk seems to be in contrast with the properties of its stellar population.

A large survey of the RR Lyrae in the Galactic bulge will allow us to map its 3-D structure (unlike the singleepoch surveys that can provide only 2-D maps) and will give us key information on the age of its population, given that RR Lyrae are tracers of the old population. This will allow us to make an important step forward in the solution of this puzzle. In the case of the bulge, the peak of their luminosity distribution defines the distance to the Galactic Center[6]. With the present project, the peak and width of the distribution can be measured with the required precision to determine the 3-D structure not only of the bulge, but also of the Sgr galaxy located behind the Milky Way[7].

At the same time, a comparison between the RR Lyrae (and type II Cepheids as well) in the field and in the globular clusters may hold precious information about the formation of the bulge. Modern ΛCDM cosmology predicts that large galaxies such as the Milky Way formed by accretion of hundreds of smaller “protogalactic fragments” perhaps not unlike the progenitors of the present-day dwarf spheroidal satellites[8]. Interestingly, two very massive globular clusters in the Galactic bulge, NGC 6388 and NGC 6441, have recently been suggested to be the remains of ancient dwarf galaxies that were accreted in the course of the Galaxy’s history[9]. These clusters might, in this sense, prove similar to the cases of M54, in the center of the Sagittarius dwarf spheroidal galaxy, which is currently being incorporated by the Milky Way[10], and of ω Cen, which has long been suspected to be the remnant nucleus of a dwarf galaxy[11].

Our proposed search for RR Lyrae and type II Cepheid stars in the Galactic bulge will reveal the presence of any debris related to the accretion events that might have left behind the present-day NGC 6388 and NGC 6441. These globular clusters are both well known to contain anomalous RR Lyrae populations, with periods that are much longer than those of known field RR Lyrae stars of similar high metallicity[12][13]. In particular, the presence of the unusually long-period (P ≥ 0.45 d) RRc (first overtone) variables, which have so far not been found in the general field but are present in large number in both these globular clusters[14], should provide the “smoking gun” for the presence of NGC 6388/NGC 6441-related debris in the general bulge field. In like vein, long-period RRab stars (fundamental pulsators) occupying the appropriate position in the period-amplitude diagram should also provide us with a strong indication of prior membership to such a protogalactic fragment.